In dental syringes that have dental material with a relatively low viscosity such as dental adhesive or low-fluidity composite resin pre-injected therein to be used in such a state, the dental material may often be injected from a discharge port of a syringe body.
When the dental material is injected, air inevitably remains within the syringe body, and as a result, there may be cases where a defined amount of dental material cannot be injected, or a desired amount of dental material cannot be discharged during actual use of the dental syringe, for example.
In view of such a problem concerning residual air within the syringe body, an ejector that is configured to discharge residual air within an injection chamber is known, the ejector including a cylinder member having a fluid ejection port formed at its front end, a plunger member, an annular concave groove that is formed at an outer periphery of the plunger member near its front end, and a sealing O-ring that is mounted at the bottom face of the annular concave groove. An air release passage is formed at the bottom face of the annular concave groove, and the annular concave groove is arranged to extend below the O-ring at a position where the O-ring comes close to the rear face of the annular concave groove. When the air pressure within the injection chamber increases at a position where the O-ring comes into contact with the rear face of the annular concave groove, the sealed state between the O-ring and the rear face is released via the air release passage and air within the injection chamber is discharged (see e.g., Patent Document 1).
The above ejector (syringe) is configured to discharge residual air to the exterior by including an air release passage. However, in practice, merely including such an air release passage may not be adequate for discharging residual air (see e.g., paragraph 0022 and FIG. 6 of Patent Document 1). Upon injecting dental restorative paste or the like (dental material) from the ejection port (discharge port), residual air is pushed by the dental restorative paste or the like (dental material) toward the O-ring (plunger) to increase the internal pressure (see e.g., paragraph 0022 of Patent Document 1), but this merely causes the plunger to be pushed toward the opposite side of the ejection port (discharge port). That is, the residual air remains sealed within the ejector (syringe).
Accordingly, in the above ejector (syringe), the rear end of the plunger is pushed by a “push plate” to fix the plunger in place so that the plunger would not move even if it were pushed by the residual air, and the air pressure within the injection chamber is increased to prompt the O-ring to move and release the sealed state between the O-ring and the annular concave groove so that the residual air can be discharged (see e.g., paragraph 0023 and FIG. 7 of Patent Document 1).
In the above ejector (syringe), the rear end of the plunger has to be pushed by the “push plate” until the residual air is discharged from the syringe, and the “plush plate” has to be removed around the time the residual air has been discharged. If the “push plate” is removed too early, the residual air may not be completely discharged. On the other hand, if the “push plate” is removed too late, the dental restorative paste or the like (dental material) may leak from the air release passage, for example. Thus, the above ejector is extremely difficult to handle.
Also, when air remains within the syringe body, even when the plunger ceases to be pushed to stop the discharge of dental material, the residual air that has been compressed up to this point may expand and push the dental material within the syringe body to cause leakage of the dental material from the discharge port of the syringe body.
In view of the problem concerning the leakage of dental material from the nozzle (discharge port) as described above, an extruding structure for a dental viscous material container having a circular internal cross-section with a uniform diameter, a nozzle arranged at its front end, and an opening arranged at its rear end is known. The extruding structure includes a moving cap that may be moved by a plunger that is inserted from the rear end of the container for pushing dental viscous material contained within the container toward the nozzle side, or an extrusion piston to which the container body is mounted. The moving cap includes a portion at the nozzle side that has a smaller diameter than the interior of the container, and an O-ring mounting groove extending from this portion into which an O-ring is mounted. The O-ring mounting groove is arranged such that a clearance between its bottom part and an inner face of the container body does not change or gets narrower, continuously or stepwise, from the nozzle side toward the opposite side (see e.g., Patent Document 2).
In the above dental viscous material container, the O-ring mounting groove is arranged such that the clearance between its bottom part and an inner face of the container body does not change or gets narrower, continuously or stepwise, from the nozzle side toward the opposite side. Accordingly, particularly in the case where the clearance gets narrower, when the plunger is pushed toward the nozzle side, the O-ring is positioned within a narrow gap and a large load is applied when the plunger is pushed toward the nozzle side, and as a result, when the plunger ceases to be pushed toward the nozzle side, the plunger moves toward the opposite side of the nozzle to reduce the load. Thus, even if the volume of residual air expands, a space within the container body (syringe body) corresponding to the amount of expansion of the residual air may be secured and the dental material may be prevented from leaking.
However, by arranging a large load to be applied upon pushing the plunger toward the nozzle side, a large force may be required upon ejecting a relatively low-viscosity dental material at the time of actual treatment, for example. Thus, the above dental viscous material container is difficult to handle, and it may be hard to discharge a precise amount of material owing to a difficulty in adjusting the force to be applied.